The length of a side that fits exactly one repetition of a texture at the default scale is 20, yes. That's what "insert normal cube" will usually give you.
I don't really know what you're asking though. If there were a button that you could click to make a side 20 units, what direction would it stretch the cube in? Would it be horizontal, vertical, or both? If it were a line, you could extend it in either of two directions, and it's not necessarily obvious which one you want there either.
Or are you asking for the "extend" option for inserting a new cube to use a length of 20 instead of what the cube you inserted from was? There are ways to work around that, but nonetheless I can see it coming in useful fairly regularly - and it's a lot less ambiguous.
More specifically: I was thinking about a button that would transform one side of the selected cube to a standard 20x20 side with the same middle point as the opposing side.